1. Field of the Invention
The present invention relates to an image display medium, and more particularly to an image display medium in which an image is reversibly displayed by the movement of charged particles upon application of an electric field. In addition, the present invention relates to an image display device using such an image display medium.
2. Discussion of the Background
Display devices such as cathode ray tubes (CRTs) and liquid crystal displays (LCDs) have been used as an image display terminal for displaying images such as letters, static images and moving images. These devices can instantaneously display images (i.e., digital data) and rewrite the images. However, these devices have various drawbacks. For, example, it is a hard work to always carry such devices. In addition, to see such a display device for a long period of time causes eye fatigue. Further, when such a display device is turned off, images therein are disappeared.
When images such as letters and static images are distributed or stored as a document, the images are typically recorded on a paper medium as a hardcopy by a printer. Images on such a hardcopy are easier to read than those in the display devices mentioned above. In addition, when reading images on hardcopies, the degree of eye fatigue is much less than that when reading images in the display devices. Further, a reader can see images on hardcopies while having a free posture. Furthermore, hardcopies can be easily carried because of having lightweight. However, hardcopies have a problem in view of material saving such that much labor and costs are necessary to recycle used hardcopies.
Therefore, a need has been existed for a paper-thin image display medium having a combination of the advantages of the above-mentioned display devices and the advantages of hardcopies. Until now, display media using a liquid crystal dispersed in a polymer, a bistable cholesteric liquid crystal, an electrochromic material, an electrophoretic material, etc. have been proposed. These media are a reflection type and therefore can display high contrast images. In addition, the media can be used as a memory display medium (i.e., the media have an image maintaining ability even when the power is off). Among these media, devices using an electrophoretic material have advantages such as good display qualities and low power consumption.
Japanese Laid-Open Patent Publication No. 5-173194 discloses an electrophoretic display medium. The display medium has a pair of parallel electrodes, at least one of which is transparent and in which a dispersion including electrophoretic particles is contained. By applying a voltage to the electrodes, the electrophoretic particles adhere to the transparent electrode, resulting in formation of an image. By applying another voltage, the particles adhered to the transparent electrode release from the electrode, resulting in erasure of the image.
Japanese Patent No. 2,612,472 (Japanese Laid-Open Patent Publication No. 1-300231) discloses an electrophoretic display medium. The device has a pair of parallel electrodes, at least one of which is transparent and in which a dispersion including electrophoretic particles is contained while the dispersion is separated into many cells (domains) by a porous spacer. A typical embodiment of the device is shown in FIG. 3.
In FIG. 3, numerals 6 and 6xe2x80x2 denote transparent substrates made of glass. Numerals 7 and 7xe2x80x2 denotes pattern electrodes formed on the substrates 6 and 6xe2x80x2, respectively. A dispersion 8 including electrophoretic particles whose color is different from the color of the dispersing medium are contained in the cell formed of the pair of substrates 6 and 6xe2x80x2. The surfaces of the electrophoretic particles are charged in the dispersing medium. When a voltage having a polarity opposite to the charge of the electrophoretic particles is applied to the electrode 7, the particles move toward the electrode 7, resulting in deposition of the particles on the electrode 7. To the contrary, when a voltage having the same polarity as the charge of the electrophoretic particles is applied to the electrode 7, the particles move toward the substrate 6xe2x80x2, and thereby the color of the dispersing medium can be seen from the side of the electrode 7. Thus, images can be reversibly displayed. When the dispersion 8 is simply contained in the cell without providing spacers 9, uneven images are often displayed due to aggregation of the electrophoretic particles and adhesion of the particles to the electrode 7. Therefore, a mesh-like or porous spacer 9 is provided in the cell to stably display images. In the display device having such a construction, it is difficult to evenly contain the dispersion into the thus separated cells. Alternatively, there occurs a trouble such that when the dispersion is contained in the cell, the properties of the dispersion often change, resulting in formation of images having undesired image qualities.
It is well known that the dispersion of the particles is stabilized by electrostatic repulsion effect and a stereo effect (i.e., an adsorption layer effect). With respect to the electrostatic repulsion effect, a DLVO theory is established. In this theory, the thickness of the electric double layer and the phase boundary potential (i.e., the so-called xcex6 potential) are important factors. Therefore, ions for forming such an electric double layer and a phase boundary potential are needed. Several investigations have been made for aqueous dispersion which can easily include such ions.
With respect to the stereo effect, a theory corresponding to the DLVO theory has not yet been established. An investigation using a non-aqueous dispersion system (e.g., petroleum solvent dispersion systems) is reported by F. A. Waite in J. Oil Col. Chem. Assoc., 54, 342 (1971). This investigation relates to a fundamental method for manufacturing a stable non-aqueous solvent dispersion. In this method, a block or graft copolymer which has both a component compatible with the particles dispersed in a dispersing medium and another component soluble in the dispersing medium is used.
Japanese Patent Publication No. 40-7047 discloses a method for manufacturing a dispersion, which utilizes the above-mentioned method. In the method, methyl methacrylate is radically polymerized in a hydrocarbon solvent in the presence of a degradable rubber to prepare a stable polymethyl methacrylate dispersion. It is considered that methyl methacrylate is graft-copolymerized with the degradable rubber because it is not considered that the polymethyl methacrylate particles adsorb the degradable rubber. It is considered that when the graft copolymer is included in a display dispersion, the insoluble part of the graft copolymer associates with the surface of particles therein, and the soluble part has a stereo effect. Thus, the dispersion stability of the particles can be maintained.
With respect to the dispersion improving method using a stereo effect, U.S. Pat. No. 5,914,806 discloses a method using pigment particles combined with a polymer dispersant by a covalent binding. In this dispersion, the dispersion stability of the particles is not satisfactory because the electrostatic repulsion effect is not exerted although the stereo effect can be exerted.
Japanese Patent Publication Ko-Hyo No. 9-500458 (i.e., WO95/02636) discloses that the dispersion stability of a dispersion can be enhanced due to both the stereo effect and the electrostatic repulsion effect caused by using a combination of a pigment having an acidic moiety with a polymer dispersant having an amino group. However, in this method pigments used for the electrophoretic particles are limited because they must have an acid therein. In addition, an acid/base dissociation hardly occurs in a non-aqueous solvent, and therefore only little electrostatic repulsion effect can be obtained.
Because of these reasons, a need exists for an image display medium which can reversibly display images having high contrast and resolution and which has an image maintaining ability and high response. In addition, a need exists for an image display medium which has a high flexibility in material selection and a good stability so as to be practically used for a long period of time.
Accordingly, an object of the present invention is to provide an image display medium which can reversibly display images having high contrast and resolution and which has an image maintaining ability and high response.
Another object of the present invention is to provide an image display medium which has a high flexibility in material selection and a good stability so as to be used for a long period of time.
Yet another object of the present invention is to provide an image display device which reversibly display images having high contrast and resolution and which has an image maintaining ability and high response so as to be practically used for a long period of time.
Briefly these objects and other objects of the present invention as hereinafter will become more readily apparent can be attained by an image display medium for displaying an image by electrophoresis, including a pair of electroconductive layers, at least one of which is light-transmissive and which are opposed each other to form a cell, and a dispersion contained in the cell, wherein the dispersion includes a colored particulate material having an acidic group or a basic group, a hydrocarbon solvent, a resin which is soluble in the hydrocarbon solvent and which has a basic group or an acidic group, and a compound having a nonionic polar group, and wherein when the colored particulate material has an acidic group, the resin has a basic group, and when the colored particulate material has a basic group, the resin has an acidic group.
Preferably the resin further has a nonionic polar group and/or the colored particulate material further has a nonionic polar group.
The colored particulate material preferably has a particle diameter of from 0.1 to 10 xcexcm. In addition, the colored particulate material preferably includes a colorant and a resin having an acidic group or a basic group, wherein the weight ratio (C/R) of the colorant (C) to the resin (R) is 0.1/100 to 1000/100.
In addition, the dispersion preferably includes water in an amount of from 100 to 2000 ppm.
In another aspect of the present invention, an image display device is provided which includes the image display medium mentioned above and at least one of a voltage applicator and a connector through which a voltage is applied to the device.
These and other objects, features and advantages of the present invention will become apparent upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.